The present invention relates to a simple, reliable, energy-efficient vulcanizing system for mass production of top quality anomaly-free rubber tires.
For decades it has been recognized that the performance of truck and passenger car tires is greatly improved when the tires are vulcanized with high internal curing pressures. However, as steam pressures are increased substantially above 200 psig, the high vulcanization temperatures create problems. It is difficult to provide adequate internal bladder pressure with high-temperature steam without degrading the rubber or forming an inferior tire. For many years pneumatic rubber tires have been vulcanized using internal steam pressures of around 200 psig, but this pressure is inadequate to assure proper diffusion of entrapped air in modern tires.
Many different processes have been employed to provide the desired high internal bladder pressure while limiting the vlucanizing temperature, as disclosed, for example, in U.S. Pat. Nos. 2,066,265; 3,489,833; 3,579,626 and 4,027,543; and in "The Applied Science of Rubber", W. J. S. Naunton (1961), pages 1053-1083. The latter publication describes various tire curing systems which have been used including those employing circulating hot water, dead-end hot water, and steam. Some use high-pressure steam in the bladder followed by lower pressure steam; some use high-pressure steam followed by high-pressure dead-end hot water; and others use high pressure steam followed by circulating high-pressure hot water.
The circulating hot-water curing systems with internal bladder pressures of 300 to 400 psig provide tires of highest quality, the so-called "anomaly-free" tires which are free from defects due to entrapped air and detectable by holographic testing equipment. Similar results can be obtained in steam-nitrogen systems using nitrogen at pressures of 300 to 450 psig (see U.S. Pat. No. 4,027,543). Both of these systems provide the temperature-pressure combination needed to provide top-quality truck and passenger car tires, but they require large capital investment and are costly to operate and maintain.
Dead-end water curing systems have been known for decades and can be used for vulcanization of tires and other rubber products in cures of limited duration. In this system heated water at a pressure of 300 psig or higher is supplied to the curing bladder within the mold and the outward flow is interrupted or cut off while the inlet pressure is maintained. When used for curing tires the system has a number of problems. Still water results in poor heat transfer because of the low film coefficient which is less than 2 percent of that for steam. Temperature stratification begins at the water-bladder interface in a short period of time. Because of the temperature gradient, the bladder temperature at one sidewall of the tire is different from that at the other side. Also the temperature of the water drops over a period of time as heat is transferred to the bladder. Curing of the tire is not uniform.
Circulating hot water systems do not suffer from these problems and have been considered more reliable. Water circulation increases the rate of heat transfer to the bladder and makes it possible to maintain a constant vulcanizing temperature. Because of the superiority of the circulating hot water curing system and the high standards demanded by the tire industry, this system was chosen by tire manufacturers and has been the mainstay of the tire industry for decades in spite of its inefficiency and high cost of operation and maintenance (see U.S. Pat. No. 3,579,626).
The rubber tire industry has long believed that vulcanization at high pressures, such as 300 to 400 psig, was necessary to manufacture anomaly-free tires and that there was no simple and reliable way to maintain tire quality without costly equipment.
Heretofore, heavy duty truck tires, for example, have been cured for 40 to 90 minutes or more using circulating water at a high pressure, such as 400 psig, and a constant temperature, such as 170.degree. C. to 185.degree. C. Water circulation was considered necessary in such cures to meet quality standards. Prior to this invention it was not known how those high standards could be met without great capital investment in new equipment or how they could be met with a simple change in procedure using dead-end water or steam to provide the internal bladder pressure during curing.
Tire manufacturers have long recognized the need to minimize the mold curing time in making tires and the fact that millions of dollars could be saved every year by merely reducing the time for curing each tire by one minute (min) or so. The fact is that a 0.5 min reduction in cure time is extremely important in the vulcanization of passenger car tires where the ordinary curing cycle may be 15 min or less. Major efforts have been made over the years to reduce the cure time by use of new rubber compositions and higher curing temperatures. However, prior to this invention, it was not known how to provide a cure with standard rubber tire compositions which maintained or improved tire quality while reducing cure time 5 to 10 percent.